Ever since Rockstar announced there would be new Grand Theft Auto 4 content available for download in February, fans have been champing at the bit to return to Liberty City. Given their flagship franchise's rabid fan base, Rockstar could have probably gotten away with an expansion pack which included a few new vehicles and weapons and maybe a couple of new environments or arenas.

But The Lost & Damned – the first of two planned 'episodes' set for release exclusively for Xbox 360 consoles – is far more than that; in short, players willing to splash out 1600 points on Xbox Live (which can be bought for around £15) will be rewarded with a new game within a game.

Aside from the new weapons, vehicles, clothes and mini games, The Lost & Damned content boasts around 20 brand new missions along with a whole new plot which features a whole new cast of characters.

Players take on the role of Johnny Klebitz (first seen in the GTA 4 mission, "Blow Your Cover"), the vice president of a gang of bikers called The Lost. Johnny has been leading the gang ever since their psychotic leader, Billy, was sent to rehab and under his stewardship, The Lost has started to become a more focused and profitable (if still, highly illicit) organisation. They've even brokered a peace deal of sorts with their old rivals, The Angels Of Death.

But the uneasy calm unravels with the release of Billy. Not only does he immediately destroy the treaty by killing his opposite number in the Angels, he then proceeds to ignite a vicious gang war which threatens to destroy all of Johnny's good work while attracting the attention of every cop in Liberty City. Hilarity inevitably ensues.

The Lost & Damned's story runs in tandem to the main plot of GTA 4 and occasionally overlaps it, but it's very different in terms of style and pacing. Rather than slowly exploring Liberty City through the eyes of an immigrant with a helpful series of tutorial missions (as in GTA 4), players step into the shoes of a lifelong native and are hurled, all guns blazing, straight into the action.

"Because we know that people have at least played some of GTA 4, we don't want to start again with another introduction because we don't need to," says Dan Houser, writer on both GTA 4 and the downloadable episodes. "You're a native of the city yourself by then and you want to be thrust into the story a bit more."

In the three missions we played, the action came fast and furious. In the first, we had to chase down adversaries while tearing through the streets on a bike. The second saw a full-scale gun fight take place under a freeway off-ramp. The third, however, was arguably the most adrenaline-fuelled mission: re-calling the classic "on-rails" chases from earlier Grand Theft Auto games, it involved hurtling through the streets on the back of a bike, firing round after round into pursuing opponents, which resulted in enough cinematic explosions and white-knuckled excitement to fuel a dozen action films.

The pacing of the missions aside, The Lost & Damned also comes with a new set of game mechanics and unique features. As vice-president of The Lost, Johnny arrives with a built-in community in the form of his bike gang, and the story's main hook is that this community, and Johnny's attachment to it, are starting to fall apart.

"Johnny's story is about the gang and the criminal group of outlaws and the moral codes that he thinks that they've been living by all this time which turn about to be a load of rubbish," says Houser.

"The gang is being split into two factions. He's with the guys who just want to make money and ride off into the sunset and try and live by their code. The other faction, lead by the gang's leader Billy Gray, just want to fight to the end.

"It was fun to play around with pack animals in groups and we had a lot of fun exploring that stuff. The mechanics of having a gang with you all the time works really well from a game design standpoint, and I think the guys did an amazing job making that feel very different [from GTA 4's main campaign]."

Indeed, the sense of a splintering community is apparent right from the outset, both through the cut scenes in which Billy can be seen causing all manner of mayhem, and through the gameplay itself. When players ride on a mission with The Lost, a gang logo appears on the road indicating where players need to be in order to remain in formation. Staying in the logo gives all the riders a health boost, and you'll also hear gang members chatting to each other about the history of The Lost. It's advantageous to keep gang members alive as long as possible; dead gang members will replaced, but the new recruits arrive with less health and less skill stats than their recently deceased comrades.

Johnny also rides a unique custom-built bike – one of the many new vehicles players will gain access to – which can't be retrieved if destroyed. Many of the gang's bikes are stylishly pimped out with long front forks and fat back tyres. They also, physically, feel more robust and it's harder to unseat a rider in a crash – high speed head-on collisions notwithstanding.

In the three missions we played, Johnny gains access to a safehouse – in the form of the gang's bar headquarters – as well as a couple of new weapons including a grenade launcher, an assault shotgun, pipe bombs an a sawn-off shotgun. Rockstar says that more weapons and locations are available in the downloadable content, as well as new mini games including arm wrestling for money and card games. And of course, all of this comes wrapped in GTA 4's excellent graphics and kitted out with superb animation and top-rate voice work.

The main draw is that nothing about the downloadable content feels tacked on or recycled from GTA 4's main game. The Lost & Damned feels and plays like a stand alone entity in its own right. According to Houser, this was the intention all along; one of the reasons the new content stars a character which didn't feature too prominently was to widen the narrative.

"We didn't want it to feel like it [GTA 4] was this very tight, narrow world. It's not meant to be that [the stories] are totally intertwined with each other," he says, "They're just two different stories in this big metropolitan area that cross over each other every once in a while.

"That was the feeling we wanted rather than, 'here's the same story except from ten degrees to the left'."

So can we pre-empt the next episode by seeing which characters feature the least prominently in the background?